This invention relates to an acoustic jet having a nozzle directed essentially tangentially downstream into a flow of gas along a surface to control the boundary layer thereof.
Boundary layer separation is a fundamentally limiting mechanism which constrains the design of gas flow systems. As an example, it is known in the helicopter art that retreating blade stall (boundary layer separation from the leading edge of the rotor blade) establishes limits on rotor load and flight speed. In addition to the loss of capability to generate lift, unsteady blade stall transmits very large impulsive blade pitching moments to the flight control system. In order to prevent excess control loads, stall boundaries are set as a function of rotor load and flight speed. Stall boundaries define the maximum blade loads, which impact maneuverability and agility as well as speed and payload. Similar boundary layer separation problems affect diffusers, fans in air moving equipment and jet engines, airplane wings, other airfoils, fuselages, flow ducts, and other structures having surfaces with aerodynamic profiles.
Gas flow in the boundary layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the wall: from zero at the surface, raising to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the gas times the square of its velocity. This near-wall, low-momentum fluid can be problematic for the case where the static pressure rises along the direction of the flow. For example, along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case in a diffuser and on the suction side of an airfoil such as a fan blade or an airplane wing, the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. If the pressure rise is sufficiently large, the momentum and energy of the gas along the surface is consumed in overcoming this pressure rise, so that the gas particles are finally brought to rest and then flow begins to break away from the wall, resulting in boundary layer separation (FIG. 1A). Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift) and dramatic decrease in system efficiency, due to conversion of flow energy into turbulence, and eventually into heat. It is known that boundary layer separation can be deterred by increasing the momentum flux of the gas particles flowing near the surface. In the art, the deterrence or reduction of boundary layer separation is typically referred to as xe2x80x9cdelaying the onset of boundary layer separationxe2x80x9d.
The simplest and most common method for reducing boundary layer separation includes small vortex generators, which may typically be tabs extending outwardly from the surface (such as the upper surface of an airplane wing), which shed an array of streamwise vortices along the surface. The vortices transport the low momentum particles near the surface away from the surface, and transport the higher momentum particles flowing at a distance from the surface toward the surface, thereby improving the momentum flux of particles flowing near the surface in the boundary layer downstream of the tabs. This has the effect of deterring boundary layer separation at any given velocity and over a range of angle of attack (where the uncontrolled separation is downstream of the vortex generators). However, as is known, tab-type vortex generators create parasitic drag which limits the degree of boundary layer separation that can be efficiently/practically suppressed.
Another known approach employs continuous flow into or out of the boundary layer. A wall suction upstream of the boundary separation line (that is, the line at which the onset of full boundary layer separation occurs across the surface of an airfoil or a diffuser) simply removes low momentum flux gas particles from the flow adjacent to the surface, the void created thereby being filled by higher momentum flux gas particles drawn in from the flow further out from the surface. A similar approach is simply blowing high energy gas tangentially in the downstream direction through a slot to directly energize the flow adjacent to the surface. Both of these flow techniques, however, require a source of vacuum or a source of pressure and internal piping from the source to the orifices at the surface, which greatly increases the cost, weight and complexity of any such system. These techniques have not as yet been found to be sufficiently effective to justify use over a wide range of applications.
A relatively recent approach, so-called xe2x80x9cdynamic separation controlxe2x80x9d uses perturbations oscillating near the surface, just ahead of the separation point, as are illustrated in U.S. Pat. No. 5,209,438. These include: pivotal flaps which oscillate from being flush with the surface to having a downstream edge thereof extending out from the surface, ribbons parallel to the surface, the mean position of which is oscillated between being coextensive with the surface and extending outwardly into the flow, perpendicular obstructions that oscillate in and out of the flow, and rotating vanes (microturbines) that provide periodic obstruction to the flow, and oscillatory blowing. These devices introduce a periodic disturbance in vorticity to the flow, the vortices being amplified in the unstable separating shear layer into large, spanwise vortical structures (see FIG. 1B) which convect high momentum flow toward the surface, thereby enabling some pressure recovery. It is consistently reported in the relevant literature that at least two large coherent vortical structures must be present over the otherwise separated region for the control to be effective. Such a flow is neither attached nor separated, under traditional definitions. However, such perturbations must be actively controlled as a function of all of the flow and geometric parameters, dynamically, requiring expensive modeling of complex unsteady flow structures and/or significant testing to provide information for adapting to flow changes either through open loop scheduling or in response to feedback from sensors in the flow.
A recent variation on the dynamic separation control is the utilization of a so-called xe2x80x9csyntheticxe2x80x9d jet (also referred to as xe2x80x9cacoustic jetxe2x80x9d or xe2x80x9cstreamingxe2x80x9d) directed perpendicular to the surface upstream of the boundary separation line of the surface. This approach has been reported as being highly parameter dependent, thus also requiring dynamic control; and, the results achieved to date have not been sufficient to merit the cost and complexity thereof in any product or practical application. In Redinotis et al, xe2x80x9cSynthetic Jets, Their Reduced Order Modeling and Applications to Flow Controlxe2x80x9d, AIAA 99-1000, presented at 37th Aerospace Sciences Meeting and Exhibit, Reno, Nev., Jan. 28, 1999, a laminar flow of water (Reynolds number=6600) flowing around a half-cylinder used a tangential synthetic jet which induced natural instability of the shear layer, leading to large vortical coherent structures of the type referred to with respect to FIG. 1B, hereinbefore, which promoted mixing and momentum flux exchange between the inner and outer parts of the boundary layer. As stated therein, the process takes advantage of the Coanda effect. That requires significant local surface curvature in the vicinity, and particularly downstream, of the point of injection of the synthetic jet. Although flow separation was delayed somewhat, it was not eliminated, as shown in FIG. 17(C) therein.
Objects of the invention include: absolute adherence of a boundary layer of laminar or turbulent gaseous flow to an adjacent surface; improved boundary layer characteristics in turbulent flow; reduced boundary layer thickness; improved deterrence of gas flow boundary layer separation; increased efficiency of gas flow machinery; improved helicopter stability; improved effectiveness of fan, helicopter rotor and other blades, wings, other airfoils, fuselages and other aerodynamic structures; boundary layer control which is effective, efficient, having moderate initial cost and low operating costs; and boundary layer control which is relatively simple and provides little parasitic impact on the host structures and systems.
This invention is predicated in part on the fact that the outflowing jet stream of an acoustic jet will clear the orifice or nozzle area sufficiently before the onset of negative pressure, which therefore will cause replenishment of gas mass within the jet cavity with molecules which are other than those in the emitted jet stream, specifically, the low momentum molecules of the approaching boundary layer. This invention is also predicated in part on our discovery that an acoustic jet directed tangentially into a boundary layer of a turbulent gaseous flow will produce a net negative flow averaged over time which is generally perpendicular to the surface and a net positive flow averaged over time which is generally parallel to the surface.
According to the present invention, an acoustic jet directed at a low angle of incidence into the boundary layer of a turbulent gaseous flow provides pulses of high momentum flux particles, which are essentially the previously ingested low momentum flux particles that have been accelerated, injected substantially tangentially into the boundary layer to cause, in the region downstream of the tangential acoustic jet, an essentially steady streamline flow with the boundary layer absolutely attached to the surface. In one application of the invention, the acoustic jet is directed at a low angle of incidence in the vicinity of the boundary layer separation point of a diffuser, a flap, an airfoil, or other aerodynamic profile thereby to deter or prevent boundary layer separation. In further accord with the invention, the jet may be located at the entrance to a diffuser or at the edge of a bluff body to deter or prevent boundary layer separation.
The negative pressure portion of the acoustic jet cycle (instroke, FIG. 2A) creates a flow of low momentum flux gas particles perpendicular to the surface, entering the chamber, thereby removing low momentum flux gas particles from the approaching boundary layer, such particles being injected essentially tangentially into the boundary layer during the positive pressure portion of the acoustic jet cycle (outstroke, FIG. 2B) to provide adequate momentum flux in the boundary layer, to deter the onset of boundary layer separation downstream thereof, including (with adequate drive) absolute adherence to the adjacent surface. This action of the tangential acoustic jet of the present invention energizes the boundary layer on both the instroke and outstroke, the time average of which is shown in FIG. 2C, making the boundary layer resistant to separation during both strokes, thereby completely preventing separation. The tangential acoustic jet totally suppresses separation without the introduction of large coherent structures. In addition, the most effective frequency for the tangential acoustic jet of the present invention corresponds to a frequency where particle displacement in the nozzle is the largest, relating to acoustic streaming parameters, and occurs at low frequencies where the actuator output is designed to be maximal; this is in contrast to the dynamic separation control of the prior art (hereinbefore) in which the frequency directly depends on flow speed, length of separation, and approaching boundary layer characteristics.
The invention may be practiced utilizing cavities in which the acoustic forcing energy is applied through a resilient member or a rigid member acting as a wall of the cavity, the member being vibrated by electric, magnetic or mechanical forcing, to induce pressure oscillations in the gas at an effective frequency, such as a loudspeaker, preferably with a high Q (quality factor, a measure of mechanical losses) centered at an effective frequency for boundary layer control, or other electroacoustic or mechanoacoustic transducer, such as simple vibrators attached to diaphragms or pistons, powered by rotary or linear devices, piezoelectric drivers, and the like.
In contrast with all of the prior art, the frequency of excitation of the tangential acoustic jet of the present invention is essentially unrelated to the flow and the surface (that is, independent of flow speed, length of separation and approaching boundary layer characteristics), and is, instead, a function of the characteristics of the acoustic jet itself, including the actuator resonant frequency. A selected frequency for the invention is one which will provide the highest conversion of input power to flow output power. As described by Ingard, U. xe2x80x9cOn the Theory and Design of Acoustic Resonatorsxe2x80x9d, Journal of the Acoustical Society of America, Vol. 25, No. 6, Nov. 1953, the amplitude of excitation of the acoustic jet must be high enough so that the gas particles separate from the orifice region, roll up into ring vortices and convect away, forming the mainstream of flow, or synthetic jet. However, the effect achieved by a tangential acoustic jet in accordance with the invention is not frequency dependent, and the frequency of the jet is wholly independent of the flow which the jet is being used to control. When the gas is air, a suitable frequency may be on the order of 20 Hertz to several hundred Hertz. Other frequencies may be used to suit any particular implementation of the present invention. The amplitude for a given frequency must be high enough with tangential injection of the particles into a flow, to ensure that gas particle displacement through the nozzle is sufficient to prevent the particles from being re-entrained into the slot.
The invention is particularly advantageous since the acoustic frequency may remain fixed for flows having Reynolds numbers ranging from a few hundred to several million in contrast with any prior methodology. It is a unique solution for flows with Reynolds numbers exceeding the critical value above which the flow is turbulent.
In accordance with the invention, the nozzle is directed at as small an acute angle to the boundary layer as is practicable, referred to herein as xe2x80x9csubstantially tangentialxe2x80x9d; the angle may range from near zero degrees to about forty degrees, when necessary, while still obtaining some of the benefits of the invention.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.